Heat exchanger, combination with heat exchanger and method of manufacturing the heat exchanger

ABSTRACT

A heat exchanger for heat exchange between a first and a second fluid and comprising a cylindrical casing  2 , a cylindrical fluid conduit  5  arranged inside the casing such that an axially extending tubular space  8  is defined, at least one helical coil  9, 10  of a finned or corrugated tube being arranged inside the tubular space, and adjustable throttle means  17, 17   a   , 18  adapted and arranged for adjustably throttling flow of the first fluid through the conduit  5  to adjust the flow of the first fluid through the conduit and the first tubular space for adjusting the heat exchange between the first fluid and the second fluid flowing through the helical coils  9, 10.

The present invention relates to a heat exchanger for heat exchangebetween a first fluid and a second fluid and comprising a generallycylindrical casing with a first inlet and a first outlet for allowingsaid first fluid to flow through said casing in a generally axialdirection, and at least one helical coil of a finned or corrugated tubearranged inside said casing.

Heat exchangers of this type are known where the first fluid is forcedto flow from inside the coil or coils outwards or vice versa. Thetransfer of heat from the first fluid to the second fluid in the coilsis not very well controlled and therefore not as efficient as possible.

It is an object of the invention to provide a heat exchanger of the typein reference where the flow of first fluid, for instance exhaust gasfrom a natural gas fired turbine, an internal combustion engine, anincinerator, a furnace, a burner or the like, takes place around thefinned tube in a well controlled and efficient manner affording a highlyefficient heat transfer from the first fluid to the second fluid, forinstance water.

According to the invention this object is obtained by providing agenerally cylindrical fluid conduit inside said casing generally coaxialtherewith so that an axially extending first tubular space is definedbetween said conduit and said casing, said conduit having a second inletand a second outlet for allowing said first fluid to flow through saidconduit in a generally axial direction, and said first tubular spacehaving a third inlet and a third outlet for allowing said first fluid toflow through said first tubular space in a generally axial direction,the at least one helical coil of a finned or corrugated tube beingarranged inside said first tubular space generally coaxial therewith andhaving a fourth inlet and a fourth outlet for allowing said second fluidto flow through said finned tube.

Hereby, the first fluid is forced to flow around the finned tube coilwindings in a very efficient manner for heat exchange. This also reducesthe space requirements for the heat exchanger, the so-called“footprint”.

According to the invention, the heat exchanger further comprises firstadjustable throttle means for adjustably throttling said flow of saidfirst fluid through said conduit and/or second adjustable throttle meansfor adjustably throttling said flow of said first fluid through saidfirst tubular space.

Hereby, the flow of first fluid may by-pass the finned tube coils sothat the heat transfer to the second fluid may be reduced according tothe demand for heated second fluid. Furthermore, the pressure loss fromthe inlet of the casing to the outlet thereof may be reduced byby-passing the finned tube coils which is desirable during start up offor instance a gas fired turbine generating the first fluid in the formof exhaust gas from the gas combustion.

In one of the currently preferred embodiments of the heat exchangeraccording to the invention, said first throttle means comprise a firstbutterfly valve, preferably arranged adjacent said second inlet or saidsecond outlet, and said second throttle means comprise a secondbutterfly valve, preferably arranged adjacent said third inlet or saidthird outlet. Hereby, rather simple mechanisms that are simple to adjustand regulate are provided.

In an alternative embodiment, said first throttle means comprise a firstbutterfly valve, preferably arranged adjacent said second inlet or saidsecond outlet, and said second throttle means comprise a ring havingplanar dimensions corresponding to the cross section of said firsttubular space and being arranged for being displaced from a heatingposition wherein said flow of first fluid through said tubular space issubstantially unhindered to a bypass position wherein said flow issubstantially obstructed. In this embodiment, the space requirements arereduced.

In another currently preferred embodiment, said first and secondthrottle means comprise a fixedly arranged stationary plate with firstand second apertures provided therein arranged such that said second andthird inlets or outlets are obstructed by said plate such that the flowof first fluid through said conduit and said first tubular space takesplace through said first and second apertures, respectively, in saidstationary plate, and one or two movable plates with third and fourthapertures provided therein and arranged displaceable, preferablyrotatably displaceable, from a bypass position overlying said stationaryplate, wherein said third apertures coincide with said first aperturesand said fourth apertures do not coincide with said second apertures, toa heating position overlying said stationary plate wherein said fourthapertures coincide with said second apertures and said third aperturesdo not coincide with said first apertures. This embodiment of the firstand second throttle means requires a minimum of space and isparticularly well suited for precise adjustment of the by-pass flowthrough the conduit relative to the heat transfer flow through thetubular space.

In the currently preferred embodiment, the heat exchanger according tothe invention further comprises preferably motorized actuating means foradjusting the throttling effect of said first and second throttle means,and said throttling means and said actuating means are preferablyadapted such that substantially any rate of flow between a maximum andminimum rate of flow of said first fluid through said second inlet andsaid third inlet may be obtained. Said minimum rate is substantiallyequal to zero. Hereby, any distribution of fluid flow between theby-pass conduit and the tubular space may obtained which allows simpleand precise regulation of the output of the heat exchanger according tothe requirements for heat transfer from the first fluid. By allowingsubstantially total by-pass, no heat is transferred to the second fluidwhich is advantageous in case no heat transfer is needed to meansexterior of the heat exchanger and therefore no temperature increasewith consequent steam formation will take place in the finned coil orcoils.

The currently preferred embodiment of a heat exchanger according to theinvention comprises two or more of said helical coils arrangedconcentrically and such that mutually adjacent coils are radially spacedsuch that an axially extending second tubular space is provided betweensaid mutually adjacent coils, and the outer surface of said conduit isspaced radially from the coil adjacent said surface such that an axiallyextending third tubular space is provided between said surface and saidadjacent coil, the radial dimensions of said second and third tubularspaces being adapted so as to achieve a certain pressure loss for agiven rate of flow of said first fluid through said first tubular space.

Hereby, the pressure loss from said first inlet to said first outlet fora given flow of first fluid may be kept at a minimum while notsubstantially reducing the efficiency of the heat exchanger. This isparticularly of importance in connection with gas fired turbines beingthe origin of said first fluid because gas turbines are particularlysensitive to the back pressure at the exhaust outlet thereof.

Preferably, the mutually adjacent individual windings of a coil aremutually axially spaced such that a helically extending space isprovided between said adjacent windings. Hereby any differential thermalexpansion or contraction of the casing and/or conduit relative to thecoils in the axial direction will be taken up by said helicallyextending space.

In a currently preferred embodiment of a heat exchanger according to theinvention and comprising three or more of said helical coils arrangedconcentrically and the interior diameter of the finned tubesconstituting the coils preferably being the same, third throttling meansare provided in the tubes constituting the coils located radiallyinwards of the outermost coil for increasing the pressure loss throughthe tubes of said inner coils so as to compensate for the shorter lengthof said tubes relative to the length of the tubes of the outermost coilsuch that the rate of flow of said second fluid through the tubes of allthe coils is substantially the same for a given uniform pressure in saidsecond fluid at said fourth inlets. Hereby, the heat transfer efficiencyof each coil will be substantially the same without having to vary thediameter of the tubes of each coil to achieve this effect.

In the currently preferred embodiment of a heat exchanger according tothe invention said third throttling means are constituted by a reductionof the cross sectional area of the flow of said second fluid relative tothe internal cross sectional area of said tubes, the heat exchangerpreferably further comprising an inlet header tube and an outlet headertube in fluid communication with said fourth inlets and fourth outlets,respectively, of all said tubes through corresponding communicationapertures in said header tubes, said reduction of flow cross sectionalarea being constituted by reduced size of said communication aperturesin said inlet header tube and/or in said outlet header tube. This is aparticularly simple and inexpensive way of compensating for thedifferent lengths of the different coils.

In case the heat exchanger according to the invention is to be used forgenerating steam, then according to the invention each helical coil mayadvantageously comprise two or more helically wound finned tubesextending adjacent one another with the same pitch. Hereby the number offlow paths is increased which is advantageous in connection with thelarge volume expansion of the water in the coil tubes resulting from thesteam generation.

In another aspect, the present invention relates to a combination of aheat exchanger according to the invention and an exhaust gas generatingcombustion means such as a natural gas fired turbine, an internalcombustion engine based on gasoline, diesel oil or natural gas, afurnace, a burner, an incinerator and the like, the combinationcomprising interconnection means for interconnecting an exhaust gasoutlet of the combustion means with said second and third inlets of theheat exchanger such that said exhaust gas constitutes said first fluid.

The currently preferred embodiment of the combination according to theinvention may further comprise heat exchanging means for heat exchangebetween said second fluid and a third fluid and/or the surroundings ofsaid heat exchanging means, said heat exchanging means being in fluidcommunication with said fourth outlet, measuring means for measuring therate of heat exchange of said heat exchanging means, signal output meansfor emitting a signal representing the result of a measurement carriedout by said measuring means, and first control means for controlling theadjustment of said first and second throttle means and adapted forreceiving said signal.

Preferably, the combination according to the invention further comprisessecond control means for controlling the adjustment of said firstthrottle means such that the throttling effect thereof is at a minimumduring the start up phase of the combustion means.

In yet another aspect, the present invention relates to a method ofmanufacturing a heat exchanger according to the invention and comprisingthe steps of providing a first length of finned or corrugated tube,providing a body having a substantially circular cylindrical surface,providing rotating means for causing relative rotation of said tube andsaid surface, arranging a lead portion of said tube abutting againstsaid surface, and causing relative rotation of said surface and saidlead portion such that said first length of tube is helically wound onsaid surface to form a first helical coil. Hereby, a particularlysimple, precise and inexpensive method of manufacturing a heat exchangeraccording to the invention is achieved.

In connection with heat exchangers according to the invention having twoor more concentric coils, the method according to the inventionpreferably comprises the further steps of providing spacing means,attaching said spacing means to said first helical coil, providing asecond length of finned or corrugated tube, arranging a lead portion ofsaid second length of finned tube abutting against said spacing means,causing relative rotation of first helical coil and said lead portion ofsaid second length of tube such that said second length of tube ishelically wound on said spacing means to form a second helical coilradially spaced from said first helical coil.

So as to avoid inaccuracies in the diameter of the coils and otherdisadvantages, the method according to the invention preferablycomprises the further steps of fixating said helical coil relative tosaid body, and subjecting said body and said coil to annealing heattreatment and/or fixating said second helical coil relative to said bodyand/or said first helical coil, and subjecting said body and said firstand second coils to annealing heat treatment. Hereby the diameteralteration of the coils because of elasticity and stresses in the steelof the coil tubes is avoided in a simple and cost efficient manner.

In the following the invention will be explained more in detail withreference to different embodiments thereof shown, solely by way ofexample, in the accompanying drawings where:

FIG. 1 is an elevational partly sectional diagrammatic view of a firstcurrently preferred embodiment of a heat exchanger according to theinvention,

FIGS. 2-3 are schematic plan views illustrating a fin configuration ofthe finned tubes according to the invention,

FIG. 4 is a schematic bottom view of the embodiment of FIG. 1,

FIG. 5 is an elevational partly sectional diagrammatic view of a secondcurrently preferred embodiment of a heat exchanger according to theinvention,

FIG. 6 is a schematic view of an inlet header tube for an embodiment ofthe heat exchanger according to the invention provided with fourconcentrically arranged finned tube coils,

FIG. 7 is a broken away elevational view of a third embodiment of a heatexchanger according to the invention,

FIG. 8 is a diagrammatic enlarged scale view of a portion of theembodiment in FIG. 1 illustrating the spacing of the finned tubes of thecoils,

FIG. 9 is a schematic elevational, broken away, partly sectional view ofthe top of the embodiment shown if FIG. 5 illustrating a firstembodiment of throttle means according to the invention,

FIG. 10 is a schematic elevational, broken away, partly sectional viewof the top of a fourth embodiment of a heat exchanger according to theinvention illustrating a second embodiment of throttle means accordingto the invention,

FIG. 11 is a schematic top view of the embodiment of FIG. 10,

FIG. 12 is a schematic elevational, cut away view illustrating a thirdembodiment of throttle means according to the invention,

FIG. 13 is a schematic top view illustrating a fourth embodiment ofthrottle means according to the invention,

FIG. 14 is a schematic, partly sectional, perspectival, enlarged scaleview of the top header tube and fastening means for the coils of theembodiment of FIG. 1,

FIG. 15 is a schematic top view illustrating the method according to theinvention of manufacturing the embodiment of FIG. 5, and

FIG. 16 is a diagram illustrating one embodiment of control meansaccording to the invention for adjusting the throttle means according tothe invention.

Referring first to FIGS. 1 and 4, a heat exchanger 1 according to theinvention comprises an outer cylindrical casing 2 provided with aflanged inlet 3 and a flanged outlet 4. An interior cylindrical casingor conduit 5 having an inlet 6 and an outlet 7 is arranged coaxiallywith the outer casing 2 thereby defining a tubular space 8 wherein twocoils 9 and 10 of finned tubing are arranged. The finned tubing consistsof a tube 11 provided with fins 12 arranged generally transversely tothe axis of the tube 11.

The finned tube coils 9 and 10 are arranged mutually concentric andcoaxially with the outer and inner casings 2 and 5. A flanged outletheader tube 13 and a flanged inlet header tube 14 communicate with theinterior of the tubes 11 of the coils 9 and 10 through apertures 15 and16, respectively.

A butterfly valve 17 (by-pass valve) is pivotably arranged on a shaft 18at the outlet 7 of the conduit 5, a position wherein the valve 17 is inan intermediate position between fully closing the outlet 7 and fullyopening said outlet 7 being shown with dotted lines at 17 a.Semicircular rings 19 and 20 are arranged for abutting the rim of thebutterfly valve 17 in the closed position thereof thereby ensuring agood closing function of the valve 17.

The heat exchanger 1 is primarily intended for use in combination with anatural gas fired turbine for recuperating and utilizing the heat of theexhaust gases thereof, but may in principle be used in combination withany means producing a heated gas such as internal combustion engines,furnaces, burners, incinerators and the like.

During maximum output operation of the heat exchanger 1, the butterflyby-pass valve 17 is in the closed position shown in full lines in FIG. 1whereby all the exhaust gas from the gas turbine introduced into theinlet 3 flows through the tubular space 8 past the finned tube coils 9and 10 as indicated by the full line arrows. Water to be heated isintroduced into the coils 9 and 10 through inlet header 14 and isdischarged through apertures 15 and 16 and outlet header 4 after havingbeen heated by heat transmission from the exhaust gas through fins 12 tothe tubes 11 and thereby to the water in said tubes.

The heated water is transported to not shown exterior heat exchangemeans for transmitting some of the heat of the water to some other fluidor to the surroundings, typically radiators in a building heating systemor a district heating system.

Either during start up of the gas turbine (when the pressure lossthrough the heat exchanger 1 should be at a minimum to facilitate theturbine start up) or when the exterior heat exchange means do notrequire the full heating capacity of the heat exchanger 1, then thebutterfly valve 17 is pivoted on shaft 18 so as to allow some or all theexhaust gas from the gas turbine to flow through the internal conduit 5as indicated with dotted arrows thereby by-passing the tubular space 8and the coils 9 and 10.

Hereby, the pressure loss through the heat exchanger 1 is decreased andthe heat transmission to the water in the tubes 9 and 10 is decreased.The butterfly valve 7 can also be described as a throttling means andmay be substituted by other throttling means as described in thefollowing in connection with FIGS. 9-13.

Referring now to FIGS. 2 and 3, a strip 12 a of carbon steel islaterally cut to form tabs or fingers 12 b that are bent transverse tothe plane of the strip in alternating directions and thereafter weldedonto the surface of the tube 11 in a spiral configuration by means ofwelding seam 12 c. Hereby a very effective heat transfers from the hotexhaust gas to the fingers 12 b and thereby to the tube 11 may beachieved. Other configurations with circular plate shaped fins orcorrugations may also be employed instead of the serrated spirally woundfins shown in FIGS. 2 and 3.

Referring now to FIG. 14, the tubes 11 are welded to the upper headertube 14 around the apertures 15 and 16 thereof whereby the interior ofthe tubes 11 communicates with the interior of the header tube 14, Thecoils 9 and 10 are attached to and suspended from the outer casing 2 andthe inner conduit 5 by means of a beam 22 welded to said casing andconduit. The beam 22 is welded to two rings 23 and 24 fitting tightlyaround the fins 12 b of the coils 9 and 10. A similar attachment iscarried out at the bottom of the heat exchanger adjacent the inletheader tube 13.

Referring now to FIG. 8, the position of the coils 9 and 10 relative toone another and relative to the outer casing 2 and the inner conduit 5as well as the spacing between the windings of each coil is illustrated.

The innermost coil 10 is spaced from the outermost coil 9 by a tubularspace having a thickness or radial dimension t1, while the innermostcoil 10 is spaced from the outer surface of the conduit 5 by a tubularspace having a thickness or radial dimension t3. The outermost coil 9 isnot spaced from the inner surface of the outer casing 2, i.e. the coil 9abuts the casing 2.

The spacings t1 and t3 are chosen such that the loss of pressure throughthe tubular space 8 is maintained at a level acceptable for the optimaloperation of the gas turbine (or other hot exhaust gas generating means)delivering exhaust gas to the heat exchanger 1. The heat exchangeefficiency of the heat exchanger 1 is not substantially affected by thespacings t1 and t3. On the other hand, operational tests show that if aspacing were present between the outer casing 2 and the outermost coil9, then the efficiency of the heat exchanger 1 would be considerablyreduced. These two phenomena are at least to a certain degree owing to,on one hand, turbulent flow between the coils 9 and 10 and between theconduit 5 and the coil 10 and, on the other hand, laminar flow in atubular space between the outer coil 9 and the casing 2.

There are several parameters determining the spacings ti and t3 betweenthe coils and between the innermost coil and the conduit 5. The two mostimportant considerations or parameters are:

Exhaust Gas Pressure Drop

The exhaust gas pressure drop or loss is very dependent on the exhaustgas velocity and the geometry of the heating surface of the coilwindings. The velocity is dependent on the free gas flow cross sectionalarea (total area for the gas flow between the tubes and fins in a crosssection).

-   -   Δp=ξ·½·ρ·w2, where    -   Δp: exhaust gas pressure drop [Pa]    -   ξ: pressure drop coefficient, dependent on geometry (fin shapes,        tube diameter, inline/staggered configuration, number of        windings etc),    -   ρ: Density of the gas at mean temperature between inlet 3 and        outlet 4 [kg/m^(3])    -   w: exhaust gas velocity [m/s]

In most cases, the allowable exhaust gas pressure drop in heatexchangers and boilers after gas turbines (and engines as well) is quitelimited. For gas turbines it is extremely important to minimize theexhaust gas pressure drop as the power production on the turbine (andthus the efficiency of the turbine) is very dependent on the backpressure. In connection with the heat exchanger according to theinvention the allowable exhaust gas pressure drop is preferably limitedto be below 500 Pa (50 mm water column), giving very low exhaust gasvelocities and thus large distance between the coils (alternatively morecoils giving larger gas cross section area and larger diameter of theunit).

Heat Transfer Coefficient

In general the heat transfer coefficient should be as high as possibleto minimize the heating surface area. The heat transfer coefficient isincreased with higher exhaust gas velocities and more turbulent flow.For the heating surface chosen (serrated spiral wound fin tubes) theturbulence of the flow is very good, in general giving high heattransfer coefficient even for low exhaust gas velocities.

Designing the heat exchanger according to the invention with thespacings t1 and t3 is also advantageous from a production point of viewbecause it allows the coils to be inserted in the casing individually ascompared with coils designed to abut one another or to be nested in oneanother that must be handled and inserted as a unit comprising severalcoils.

Still referring to FIG. 8, a helically extending space is providedbetween adjacent windings of each coil 9 and 10, the thickness or axialdimension of said space being t2. This spacing t2 of the windings allowsthe casing 2 and/or the conduit 5 to thermally expand and contractaxially relative to the coils 9 and 10 without causing unacceptablestresses as any differences in such expansion or contraction is taken upby variations of the spacing t2 between the windings of the coils.

EXAMPLE

In the following, the basic technical specifications for a combinationaccording to the invention of a two coil heat exchanger according to theinvention and a gas fired turbine are listed as a non-limiting example:

Dimensions of the Heat Exchanger Height excl. inlet: 1550 mm Diameterexcl. insulation: 633 mm Insulation: 100 mm covered with galvanizedsteel plate Flue gas outlet flange: DN 450, DIN 86044 Water inlet/outletconnections: Carbon steel pipe, OD 60.5 × 3.6 mm, 2 “RGW Thickness ofcasing (inner 5 and outer 2): 5 mm Weight of heat exchanger excl. water:475 kg Weight of heat exchanger incl. water: 500 kg Outside diameter oftubes 11: 38 mm Tube material thickness: 3.6 mm Fin type: Serratedspiral wound fins Height of fins: 15 mm Fin density: 250 pcs/m Thicknessof fins: 1 mm Material, tube and fins: Carbon steel Tube configuration:Inline Number of coaxial and concentric coils: 2 Number of windings: 10Tube pitch in gas direction: 70 mm Free spacing t2 between fins on coilwindings in gas direction: 2 mm Diameter of by-pass channel (innercasing 5): 323.9 mm Length of inner casing 5, incl. by-pass valve: 860mm Centre diameter of inner coil 10: 401 mm Centre diameter of outercoil 9: 555 mm Free space t3 between inner casing 5 and fins on innercoil 10: 4.5 mm Free space t1 between fins on the two coils: 9 mm Freespace between fins on outer coil and inside of outer casing 2: 0 mm Sizeof holes 15, 16 in header 13 for coil connection (both coils) 30.8 mm

Process Data Micro Gas turbine type: HONEYWELL Parallon 75 Max. electricoutput power from gas turbine: 75 kW(e) Exhaust gas flow: 0.68 kg/sExhaust gas inlet temperature to heat exchanger: 246° C. Exhaust gasoutlet temperature from heat exchanger:  90° C. Exhaust gas pressureloss across heating surface: 300 Pa Heating capacity of heat exchanger:120 kW Water inlet temperature:  50° C. Water outlet temperature:  70°C. Water flow, approx: 1.44 kg/s Pressure drop, water side: 0.2 bar

Referring now to FIG. 5, an embodiment of a heat exchanger 31 accordingto the invention having three concentric coils 32-34 is shown with thesame reference numbers being utilized for elements similar to elementsin FIG. 1. The main difference between the FIG. 1 and FIG. 5embodiments, apart from the number of coils, is that inlet apertures 35,36 and 36 of the outlet header tube 38 are different sizes so as tocompensate for the difference in coil length between the coils 32-34 asexplained below.

As the length of the tubes 11 of the different coils 32-34 are differentand all the coils are interconnected at the outlet header 38 and at theinlet header 39, a flow distribution will be established in the coilsgiving the same pressure loss through each of the coils.

Thus, the inner coils 33 and 34 [where the second fluid (typicallywater) have shorter flow paths than in the outermost coil 32] cantransport more water than the outer coil 32. The water in the coils 33,34 and 35 will then not be heated to the same temperature and willresult in a skewed and reduced recuperation of the heat contained in thefirst fluid (for instance exhaust gas from a gas fired turbine).

It is therefore desirable that the flow rate through the coils beregulated so that best possible heat recuperation is obtained with bestpossible temperature distribution both in the water and in the exhaustgas. This is achieved by creating an extra pressure loss in the innercoils 33 and 34 relative to the outer coil 35 and each other.

This can be achieved in two manners:

-   -   By providing the tubes of the coils with different diameters.        From a practical point of view this is not desirable except in        case a large number of concentric coils are involved in which        case 2-3 different tube diameters may be acceptable.    -   By installing throttle or baffle means in the tubes or at the        inlet or outlet thereof. As can be seen in FIG. 5 and FIG. 6,        this can be achieved by providing the outlet header tube 38 with        apertures 35-37 and 40-43, respectively, having different        diameters. The diameters of the individual apertures are        determined based on the tube diameter and tube length in the        individual coils of a heat exchanger. As an example of diameters        for the four apertures shown in FIG. 6 for an inner diameter of        56 mm of all four coil tubes, aperture 40 is 56 mm, aperture 41        is 13 mm, aperture 42 is 11 mm and aperture 43 is 9 mm. Other        throttle or baffle means well known in the art may also be used        to achieve the different pressure loss coefficients for the        individual coil tubes.

Referring now to FIG. 7, the inner coil adjacent the inner conduit 5comprises two parallel wound finned tubes 50 and 51 establishing twoparallel flow paths for the second fluid (typically water) indicated bythe arrows R1 and R2.

This embodiment is intended for use for steam generation where it isnecessary to take into consideration the large volume expansion of themass inside the tubes (at the transition from liquid to vapour, water tosteam) with corresponding increase in flow rate and velocity as well aspressure loss at the inner surface of the tubes. So as to providesufficient inner flow cross sectional area in the tubes it willtherefore often be necessary to use a larger tube diameter, a largernumber of coils or provide for a larger number of flow paths in otherways.

Apart from providing many coils, more flow paths may be obtained byhaving several parallel extending windings in the same coil as shown inFIG. 7, i.e. several coils with the same coil diameter and large pitch“screwed” into each other. Hereby it is obtained that a larger innercross section area is achieved without having a large number of coilswith a large diameter of the outermost coil and therefore the outercasing 2 (large footprint). This smaller footprint or outer diameter ofthe heat exchanger entails important advantages both for the end userand during manufacture, erection and transport. The axial length orheight of the heat exchanger for a given output will of course belarger, but this does normally not represent a substantial problemduring manufacture or for the end user.

Referring now to FIGS. 9-13, various embodiments of throttle means forthrottling the flow of first fluid (typically exhaust gas) through theconduit 5 and the tubular space 8, respectively, are shown.

In the embodiment of FIG. 9, the butterfly valve 17 cooperates with aring 52 that is suspended in three steel wires 53 attached atequidistant points along the ring 52. The wires 53 extend over pulleys54 to the shaft 18.

In the situation shown with full lines, the butterfly valve 17 is closedand does not allow any exhaust gas to flow through conduit 5 while thering 52 is in its highest position in which it does not throttle theflow of exhaust gas through the tubular space 8.

In the situation shown with dotted lines, the butterfly valve 17 afunctions as a by-pass valve and allows unthrottled flow of exhaust gasthrough the conduit 5 while the ring 52 a is in its full throttleposition supported on tightening rings 55 thereby preventing flow ofexhaust gas through the tubular space 8.

The shaft 18 may be actuated manually, by an electric motor or apneumatic or hydraulic mechanism. In the simplest version, the wires arewound onto and off not shown pulleys arranged on the shaft 18 such thatrotation of the shaft 18 for opening of the butterfly valve 17automatically entails lowering of the ring 52 and vice versa.

When no heat is required by the external heat consumption meansconnected to the heat exchanger 31, then the butterfly valve 17 is inits fully open position (17 a) and the ring 52 is in its lowered fullyclosed position (52 a) so that all the exhaust gas is by-passed throughthe conduit 5. Hereby, the water in the finned tube coils is not heatedso that external cooling means to avoid overheating of this water arenot necessary.

A very simple means for regulating the heat output of the heat exchanger31 is thus provided. A temperature sensor and transmitter (not shown)may be provided in the outlet header 38 for transmitting a signal to thenot shown actuator (electric motor) for the shaft 18 so that if thetemperature measured at the outlet header does not conform to therequired temperature, then the shaft rotates in the correspondingdirection to either open or shut the by-pass valve 17. Many differentregulating circuits are conceivable depending on the requirements of theend user and the configuration of the external heat consumption devicesconnected to the heat exchanger 31.

Referring now to FIGS. 10 and 11 showing an elevational and top view,respectively, of a second embodiment of the first and second throttlingmeans for the flow of exhaust gas through the internal conduit and thetubular space, respectively, the internal conduit 5 is connected to afurther conduit 56 having an outlet 57 in which a butterfly valve 58 isrotatably mounted on a shaft 59. The outlet 57 communicates with theoutlet 4 of the casing 5. The butterfly valve 58 may rotate with theshaft 59 from the shown closed position wherein the outlet 57 is totallyobstructed and an open position wherein flow of exhaust gas throughoutlet 57 is unhindered.

The tubular space 8 communicates with a space 60 defined by an extensionof the outer casing 2, said space communicating with the outlet 4through an aperture 61 in a plate 62. A butterfly valve 63 is mounted insaid aperture 61 on the shaft 59 such that rotation of the shaft 59rotates the butterfly valve 63 from the shown fully open position inwhich flow of exhaust gas from the tubular space 8 through the space 60and through the aperture 61 is unhindered to a fully closed position inwhich flow of exhaust gas through the aperture 61 is totally obstructed.The shaft 59 is connected to an electric motor 64 for being rotated inopposite directions so as to rotate the valves 58 and 63 between the twopositions described above and to any intermediate position.

Referring now to FIG. 12, in this embodiment the butterfly valves 58 and63 of the embodiment in FIGS. 10 and 11 have been substituted by abutterfly valve 65 and two butterfly valves 66, respectively, theoperation of the valves 65 and 66 and the shaft 59 being the same asdescribed in connection with the embodiment of FIGS. 10 and 11.

Referring now to FIG. 13, a stationary circular plate 70 is arrangedhorizontally in an embodiment of the heat exchanger similar to the oneshown in FIG. 1 or FIG. 4 (without the butterfly valve 17) over theoutlet of the conduit 5 and the annular outlet of the tubular space 8.The plate 70 is provided with apertures 71 communicating with theinterior of conduit 5 and apertures 72 communicating with the tubularspace 8.

A rotatably arranged circular plate 73 is arranged on top of plate 70 ona pivot 74. The plate 73 is provided with apertures 75 identical inshape and distribution to apertures 72 in plate 70 and with apertures 76identical in shape and distribution to apertures 71 in plate 70. Anelectrical motor 77 is arranged for rotating the rotatable plate 73 inboth directions.

In the position of the rotatable plate shown in FIG. 13 the apertures 71and 76 coincide or overly each other so that exhaust gas can flowpractically unhindered through the conduit 5 underlying these coincidingapertures while flow through the tubular space 8 is obstructed becausethe apertures 72 and 75 do not communicate with each other at all. Byrotating the plate 73 by means of the motor 77, a position thereof maybe attained where flow through the tubular space is relativelyunhindered because the apertures 72 and 75 coincide and flow through theconduit 5 is totally obstructed because the apertures 71 and 76 do notcommunicate with each other at all.

The lower plate 70 may instead be the rotatable one whereby the pressurefrom the exhaust gas will press the plate 70 against the stationaryplate 73 and enhance the sealing effect of abutment of the plates 70 and73 against each other. Sealing between the plates may also be achievedin many other ways obvious to those skilled in the art.

Referring now to FIG. 15, a method according to the invention ofmanufacturing a heat exchanger (the embodiment of FIG. 5) according tothe invention is illustrated.

A cylindrical body 80 is constituted by a steel plate 81 with athickness of 10 mm, a steel rod 82 inserted between the free axiallyextending edges of the plate 81, and not shown circumferentiallyextending tightening straps or wires for holding the plate 81 and rod 82in the shown cylindrical configuration.

The inner coil 34 is wound helically around the body or core 80, theleading end (not shown) and the trailing end 11 a of the pipe 11 of thecoil 34 being attached to the body by means of brackets or rods 83welded to said ends and to the body 80. The tightening straps mentionedabove are located outside the area of the body 80 to be covered by thecoil 34.

Four cylindrical plates 84 having a quarter circle cross section and athickness equal to the required radial dimension t1 (see FIG. 8) arearranged on the outer surface of coil 33 with rods 85 arranged betweenmutually adjacent axially extending edges of the plates 84. The plates84 and rods 85 are held in place by not shown circumferentiallyextending tightening straps or wires. The rods 85 have an oval orelliptical cross section and are placed between the plates 84 such thatthe major cross sectional dimension of the oval or ellipse is tangentialto the circular circumference of the plates 84.

The next coil 33 is wound helically around the plates 84 and rods 85with the leading and trailing ends of the coil 33 being welded to one ofthe plates 84 by means of brackets or rods 83 in a manner very similarto the winding and attachment of the inner coil 34.

The process is repeated for the next coil 32. If more coils than threeare required, the process described above may be repeated for any suchfurther coils.

The unit comprising the body 80, the coils 32-34 and the plates 84 withrods 85 is thereafter subjected to annealing heat treatment for avoidingelastic diameter expansion of the coils and to remove potentiallydamaging stresses.

After annealing, the attachments by means of brackets 83 are removed,the rods 85 are rotated such that the major dimension of the oval orelliptical cross section is oriented radially whereby the coils 32-34are forced to expand slightly such that the cylindrical plates 84 may beremoved. Finally rod 82 is removed such that the body 80 may be removed.The coils 32-34 are now ready for being inserted in a casing 2 with aconduit 5 placed inside the inner coil 34.

A heat exchanger according to the invention is particularly well suitedfor use in a combination or system comprising a gas fired turbine (or aninternal combustion engine utilizing natural gas as fuel). Such a systemis furthermore particularly well suited for (but not in any way limitedto) for use in a system for small scale combined production ofelectricity and heat, for instance for large buildings, hospitals, smalldistrict heating systems and the like.

Referring now to FIG. 16, a system or combination according to theinvention including a heat exchanger according to the invention, a gasfired turbine and external heat consuming devices is shown with thefollowing characteristics: Item Component Description 101 Heat exchangeraccording to the invention. 102 Exhaust gas by-pass damper or Can beregulated manually or as shown valve (butterfly valve for instance)here: regulated by an actuator (electrical motor), item 11 103 Exhaustgas stack 104 Gas fired turbine Could be another kind of componentproducing exhaust gas 105 Circulation pump A forced circulation system,to circulate the required water/fluid flow. The pressure drop on waterside in the heat exchanger, valves and piping system to be taken intoaccount when calculating the delivery head of the pump. 106 Expansiontank To take the expansion/contraction of the fluid in the system, whenthe temperature varies. 107 External end user Or other heat consumptiondevice. Can be Heat exchanger for heating use in buildings, ingreenhouses, district heating systems etc. 108 Safety valve To be openedif the pressure in the system becomes too high 109 Stop valves Normallyopen. Possible to close in case of repair. 110 Air venting valve 111Electrical motor Electrical motor, automatically controlled by signalsfrom the temperature transmitter 113. Opens the by-pass valve 102 alittle, if the water temperature becomes too high, closes the valve 102a little if the water temperature becomes too low. Set points to bedecided by the end user. 112 Drain valve To drain the water/fluid fromthe system 113 Temperature transmitter Sends signal to electrical motorif temperature measured is too high or too low

The system can comprise other exhaust gas generating devices, and theregulation of the heat exchanger's by-pass valve (and the heatexchanger's throttle valve for throttling fluid flow through the tubularspace containing the finned tube coils) can be provided for in manydifferent manners depending on the configuration of the end user's heatconsuming devices.

1. A heat exchanger for heat exchange between a first fluid and a secondfluid and comprising: a generally cylindrical casing with a first inletand a first outlet for allowing said first fluid to flow through saidcasing in a generally axial direction, a generally cylindrical fluidconduit arranged inside said casing generally coaxial therewith so thatan axially extending first tubular space is defined between said conduitand said casing, said conduit having a second inlet and a second outletfor allowing said first fluid to flow through said conduit in agenerally axial direction, and said first tubular space having a thirdinlet and a third outlet for allowing said first fluid to flow throughsaid tubular space in a generally axial direction, at least one helicalcoil comprising a tube selected from the group consisting of a finnedtube and a corrugated tube arranged inside said first tubular spacegenerally coaxial therewith and having a fourth inlet and a fourthoutlet for allowing said second fluid to flow through said tube.
 2. Aheat exchanger according to claim 1 further comprising first adjustablethrottle means for adjustably throttling said flow of said first fluidthrough said conduit and second adjustable throttle means for adjustablythrottling said flow of said first fluid through said first tubularspace.
 3. A heat exchanger according to claim 2, wherein said firstthrottle means comprise a first butterfly valve, arranged adjacent oneof said second inlet and said second outlet, and said second throttlemeans comprise a second butterfly valve, arranged adjacent one of saidthird inlet and said third outlet.
 4. A heat exchanger according toclaim 2, wherein said first throttle means comprise a first butterflyvalve, arranged adjacent one of said second inlet and said secondoutlet, and said second throttle means comprise a ring having planardimensions corresponding to the cross section of said first tubularspace and being arranged for being displaced from a heating positionwherein said flow of first fluid through said tubular space issubstantially unhindered to a bypass position wherein said flow issubstantially obstructed.
 5. A heat exchanger according to claim 2,wherein said first and second throttle means comprise: a fixedlyarranged stationary plate with first and second apertures providedtherein arranged such that said second and third inlets or outlets areobstructed by said plate such that the flow of first fluid through saidconduit and said first tubular space takes place through said first andsecond apertures, respectively, in said stationary plate, and at leastone movable plate with third and fourth apertures provided therein andarranged so as to be displaceable from a bypass position overlying saidstationary plate, wherein said third apertures coincide with said firstapertures and said fourth apertures do not coincide with said secondapertures, to a heating position overlying said stationary plate whereinsaid fourth apertures coincide with said second apertures and said thirdapertures do not coincide with said first apertures.
 6. A heat exchangeraccording to any of the claims 2-5 and further comprising actuatingmeans for adjusting the throttling effect of said first and secondthrottle means.
 7. A heat exchanger according to claim 6, wherein saidthrottling means and said actuating means are adapted such thatsubstantially any rate of flow between a maximum and minimum rate offlow of said first fluid through said second inlet and said third inletmay be obtained.
 8. A heat exchanger according to claim 7, wherein saidminimum rate is substantially equal to zero.
 9. A heat exchangeraccording to claim 1 and comprising at least two coil arrangedconcentrically and such that mutually adjacent coils are radially spacedsuch that an axially extending second tubular space is provided betweensaid mutually adjacent coils.
 10. A heat exchanger according to claim 1,wherein the outer surface of said conduit is spaced radially from thecoil adjacent said surface such that an axially extending third tubularspace is provided between said surface and said adjacent coil.
 11. Aheat exchanger according to claim 9 or 10, wherein the radial dimensionsof said second and third tubular spaces are adapted so as to achieve acertain pressure loss for a given rate of flow of said first fluidthrough said first tubular space.
 12. A heat exchanger according toclaim 1, wherein the coil has mutually adjacent individual windings thatare mutually axially spaced such that a helically extending space isprovided between said adjacent windings.
 13. A heat exchanger accordingto claim 1 and comprising three or more helical coils arrangedconcentrically, each comprising a finned tube one of the coils being aninnermost coil and another of the coils being an outermost coil, theinterior diameter of the finned tubes constituting the coils being thesame, wherein third throttling means are provided in the tubesconstituting the coils located radially inwards of the outermost coilfor increasing the pressure loss through the tubes of the remainingcoils so as to compensate for the shorter length of said tubes relativeto the length of the tubes of the outermost coil such that the rate oflow of said second fluid through the tubes of all the coils issubstantially the same for a given uniform pressure in said second fluidat said fourth inlets.
 14. A heat exchanger according to claim 13,wherein said third throttling means are constituted by a reduction ofthe cross sectional area of the flow of said second fluid relative tothe internal cross sectional area of said tubes.
 15. A heat exchangeraccording to claim 14 and further comprising an inlet header tube and anoutlet header tube in fluid communication with said fourth inlets andfourth outlets, respectively, of all said tubes through correspondingcommunication apertures in said header tubes, said reduction of flowcross sectional area being constituted by reduced size of saidcommunication apertures in one of said inlet header tube and said outletheader tube.
 16. A heat exchanger according to claim 1, wherein thehelical coil comprises two or more helically wound finned tubesextending adjacent one another with the same pitch.
 17. A combination ofa heat exchanger according to claim 1 and an exhaust gas generatingcombustion means selected from the group consisting of at least one of anatural gas fired turbine, an internal combustion engine, a furnace, aburner, an incinerator, the combination comprising interconnection meansfor interconnecting an exhaust gas outlet of the combustion means withsaid second and third inlets of the heat exchanger such that saidexhaust gas constitutes said first fluid.
 18. A combination according toclaim 17 and further comprising heat exchanging means for heat exchangebetween said second fluid and at least one of a third fluid and thesurroundings of said heat exchanging means, said heat exchanging meansbeing in fluid communication with said fourth outlet, measuring meansfor measuring the rate of heat exchange of said heat exchanging means,signal output means for emitting a signal representing the result of ameasurement carried out by said measuring means, and first control meansfor controlling the adjustment of said first and second throttle meansand adapted for receiving said signal.
 19. A combination according toclaim 17 or 18 and further comprising second control means forcontrolling the adjustment of said first throttle means such that thethrottling effect thereof is at a minimum during the start up phase ofthe combustion means.
 20. A combination of a heat exchanger for heatexchange between a first fluid and a second fluid and an exhaust gasgenerating combustion means, the heat exchanger comprising: a generallycylindrical casing with a first inlet and a first outlet for allowingsaid first fluid to flow through said casing in a generally axialdirection, a generally cylindrical fluid conduit arranged inside saidcasing generally coaxial therewith so that a axially extending firsttubular space is defined between said conduit and said casing, saidconduit having a second inlet and a second outlet for allowing saidfirst fluid to flow through said conduit in a generally axial direction,and said first tubular space having a third inlet and a third outlet forallowing said first fluid to flow through said tubular space in agenerally axial direction, and at least one helical coil comprising atube selected from the group consisting of a finned tube and acorrugated tube arranged inside said first tubular space generallycoaxial therewith and having a fourth inlet and a fourth outlet forallowing said second fluid to flow through said finned tube, thecombination comprising interconnection means for interconnecting anexhaust gas outlet of the combustion means with said second and thirdinlets of the heat exchanger such that said exhaust gas constitutes saidfirst fluid.
 21. A combination according to claim 20 further comprisingfirst adjustable throttle means for adjustably throttling said flow ofsaid first fluid through at least one of said conduit and secondadjustable throttle means for adjustably throttling said flow of saidfirst fluid through said first tubular space.
 22. A combinationaccording to claim 21, wherein said first throttle means comprise afirst butterfly valve, arranged adjacent one of said second inlet andsaid second outlet, and said second throttle means comprise a secondbutterfly valve, arranged adjacent one of said third inlet and saidthird outlet.
 23. A combination according to claim 21, wherein saidfirst throttle means comprise a first butterfly valve, arranged adjacentsaid one of second inlet and said second outlet, and said secondthrottle means comprise a ring having planar dimensions corresponding tothe cross section of said first tubular space and being arranged forbeing displaced from a heating position wherein said flow of first fluidthrough said tubular space is substantially unhindered to a bypassposition wherein said flow is substantially obstructed.
 24. Acombination according to claim 21, wherein said first and secondthrottle means comprise: a fixedly arranged stationary plate with firstand second apertures provided therein arranged such that either saidsecond and third inlets or said second and third outlets are obstructedby said plate such that the flow of first fluid through said conduit andsaid first tubular space takes place through said first and secondapertures, respectively, in said stationary plate, and at least onemovable plate with third and fourth apertures provided therein andarranged displaceable from a bypass position overlying said stationaryplate, wherein said third apertures coincide with said first aperturesand said fourth apertures do not coincide with said second apertures, toa heating position overlying said stationary plate wherein said fourthapertures coincide with said second apertures and said third aperturesdo not coincide with said first apertures.
 25. A combination accordingto claim 21 and further comprising actuating means for adjusting thethrottling effect of said first and second throttle means.
 26. Acombination according to claims 21, wherein said throttling means andsaid actuating means are adapted such that substantially any rate offlow between a maximum and minimum rate of flow of said first fluidthrough said second inlet and said third inlet may be obtained.
 27. Acombination according to claim 26, wherein said minimum rate issubstantially equal to zero.
 28. A combination according to claim 21 andcomprising two or more helical coils arranged concentrically and suchthat mutually adjacent coils are radially spaced such that an axiallyextending second tubular space is provided between said mutuallyadjacent coils.
 29. A combination according to claim 21, wherein theouter surface of said conduit is spaced radially from the coil adjacentsaid surface such that an axially extending third tubular space isprovided between said surface and said adjacent coil.
 30. A combinationaccording to claim 28, wherein the radial dimension of said secondtubular space is adapted so as to achieve a certain pressure loss for agiven rate of flow of said first fluid through said first tubular space.31. A combination according to claim 29, wherein the radial dimension ofsaid third tubular space is adapted so as to achieve a certain pressureloss for a given rate of flow of said first fluid through said firsttubular space.
 32. A combination according to claim 21, wherein themutually adjacent individual windings of a coil are mutually axiallyspaced such that a helically extending space is provided between saidadjacent windings.
 33. A combination according to claim 21 andcomprising three or more helical coils arranged concentrically, one ofsaid coils being an outermost coil, each of the coils comprising afinned tube, the interior diameter of the finned tubes being the same,wherein third throttling means are provided in the tubes of the coilslocated radially inward of the outermost coil for increasing thepressure loss through the tubes of said radially inward located coils soas to compensate for the shorter length of said tubes of said radiallyinward located coils relative to the length of the tubes of theoutermost coil such that the rate of low of said second fluid throughthe tubes of all the coils is substantially the same for a given uniformpressure in said second fluid at said fourth inlets.
 34. A combinationaccording to claim 33, wherein said third throttling means areconstituted by a reduction of the cross sectional area of the flow ofsaid second fluid relative to the internal cross sectional area of saidtubes.
 35. A combination according to claim 34 and further comprising aninlet header tube and an outlet header tube in fluid communication withsaid fourth inlets and fourth outlets, respectively, of all said tubesthrough corresponding communication apertures in said header tubes, saidreduction of flow cross sectional area being constituted by reduced sizeof said communication apertures in one of said inlet header tube andsaid outlet header tube.
 36. A combination according to claim 21,wherein a helical coil comprises two or more helically wound finnedtubes extending adjacent one another with the same pitch.
 37. Acombination according to claim 21 and further comprising heat exchangingmeans for heat exchange between said second fluid and at least one of athird fluid and the surroundings of said heat exchanging means, saidheat exchanging means being in fluid communication with said fourthoutlet, measuring means for measuring the rate of heat exchange of saidheat exchanging means, signal output means for emitting a signalrepresenting the result of a measurement carried out by said measuringmeans, and first control means for controlling the adjustment of saidfirst and second throttle means and adapted for receiving said signal.38. A combination according to claim 21 and further comprising secondcontrol means for controlling the adjustment of said first throttlemeans such that the throttling effect thereof is at a minimum during thestart up phase of said combustion means.
 39. A combination according toclaim 20, wherein said exhaust gas generating combustion means is chosenfrom the group comprising a natural gas fired turbine, an internalcombustion engine, a burner, a furnace and an incinerator.
 40. A methodof manufacturing a heat exchanger according to claim 1 and comprisingthe steps of: providing a first length of tube selected from the groupconsisting of a finned tube and a corrugated tube, providing a bodyhaving a substantially circular cylindrical surface, providing rotatingmeans for causing relative rotation of said tube and said surface,arranging a lead portion of said tube abutting against said surface,causing relative rotation of said surface and said lead portion suchthat said first length of tube is helically wound on said surface toform a first helical coil.
 41. A method according to claim 40 andcomprising the further steps of: providing spacing means, attaching saidspacing means to said first helical coil, providing a second length oftube selected from the group consisting of a finned tube and acorrugated tube, arranging a lead portion of said second length of tubeabutting against said spacing means, causing relative rotation of firsthelical coil and said lead portion of said second length of tube suchthat said second length of tube is helically wound on said spacing meansto form a second helical coil radially spaced from said first helicalcoil.
 42. A method according to claim 40 and comprising the furthersteps of: fixating said helical coil relative to said body, andsubjecting said body and said coil to annealing heat treatment.
 43. Amethod according to claim 41 and comprising the further steps of:fixating said second helical coil relative to at least one of said bodyand said first helical coil, and subjecting said body and said first andsecond coils to annealing heat treatment.